This invention relates to a method for precisely estimating the gel content, one of important characteristics, of a propylene block copolymer based on designed conditions of manufacture without involving actual production of the copolymer.
A block copolymer, which is one of typical polypropylene products, has been of wide use for its excellent impact resistance. A block copolymer is manufactured through two-stage polymerization; former stage polymerization and latter stage polymerization. In the former stage polymerization, propylene is polymerized substantially alone in the presence of a solid catalyst component to produce polypropylene particles having the solid catalyst component inside. In the latter stage polymerization, propylene is copolymerized with other xcex1-olefins, usually ethylene, in the presence of the polypropylene particles. The copolymerization is carried out by the catalytic action of the solid catalyst component contained in the polypropylene particles produced in the former stage polymerization. Accordingly, the individual block copolymer particles are made up the homopolymer portion produced by the former stage polymerization and the copolymer portion produced by the latter stage polymerization.
A gel content is one of the important characteristics of a propylene block copolymer. The term xe2x80x9cgelxe2x80x9d denotes a component which precipitates like foreign matter when a block copolymer is melt-kneaded and molded into a film. The gel is considered to be the portion that has not been melted to become uniform by the melt kneading.
Gel not only impairs the appearance of the product produced from a block copolymer but acts as a stress concentrator to reduce the product strength. Hence it has been demanded to minimize gel formation in the manufacture of propylene block copolymers.
However, no reports has been made to elucidate the mechanism of gel formation. The amount of gel produced under designed polymerization conditions could be qualitatively predicted by experience but not quantitatively. It has therefore been a conventional way in the practice that production conditions are decided so as to result in a desired gel content after a process of trial and error comprising confirming gel contents through experimental sample preparation, altering conditions of experimental preparation, and repeating sample preparation. Because this way of decision requires a great deal of time and labor, improvement has been awaited. Accordingly, this invention is to provide a method for quantitatively estimating a gel content of a propylene block copolymer based on assumed production conditions.
The present inventors have investigated the mechanism of gel formation and found as a result that gel has its origin in the copolymer portion of block copolymer particles, that is, those particles having a rubber component portion in a large proportion. Therefore, the amount of gel produced will be estimated only if the concentration of the rubber component of individual block copolymer particles can be estimated from a polymerization apparatus.
Propylene block copolymer particles are produced by polymerization and copolymerization on a solid catalyst component, and the proportions of the homopolymer and of the copolymer in the individual particles depend on the operating conditions of the polymerization apparatus and the residence times of the polymer particles in the former and the latter stages of polymerization.
Therefore, the gel content of a propylene block copolymer can be estimated by computing the rubber component concentration distribution in the block copolymer particles to be produced based on the operating conditions of a polymerization apparatus, a growth model equation representing the state of polymer particles growing on a solid catalyst component, and a residence time distribution of polymer particles in former and latter polymerization stages.